The majority of cancer deaths occur as a result of recurrent or metastatic disease rather than from the effects of the primary tumor. A tumor can be viewed as an aberrant organ initiated by a tumorigenic cancer cell that acquired the capacity for indefinite proliferation through accumulated mutations. In this view of a tumor as an abnormal organ, the principles of normal stem cell biology can be applied to better understand how tumors develop and disseminate. Many observations suggest that analogies between normal stem cells and tumorigenic cells are appropriate. Both normal stem cells and tumorigenic cells have extensive proliferative potential and the ability to give rise to new (normal or abnormal) tissues. Tumorigenic cells can be thought of as cancer stem cells (CSC) that undergo an aberrant and poorly regulated process of organogenesis analogous to what normal stem cells do. Both tumors and normal tissues are composed of heterogeneous combinations of cells, with different phenotypic characteristics and different proliferative potentials.
Cancer stem cells are believed to be a small fraction of tumor cells with stem cell-like properties, which initiate and maintain neoplastic clones. These cells have the ability to self-renew, but also give rise to progenitors that yield phenotypically diverse cancer cells but with lower tumorigenic potential. This subpopulation of stem-like cells should be highly efficient at tumor formation as compared to tumor cells that are not cancer stem cells.
Cancer stem cells (CSCs) have now been identified in a wide variety of cancers including melanomas, medulloblastomas, colon, liver, lung, prostate, breast and ovarian tumors. While CSCs do not necessarily arise from normal stem cells, they have frequently been isolated by using markers found in normal stem cells. For example, the marker CD133 has been used to identify normal adult hematopoietic and neural stem cells. CD133 has now been successfully used to enrich for CSCs from melanoma, medulloblastoma, colon and prostate tumors.
The presence of cancer stem cells has profound implications for cancer therapy. At present, all of the phenotypically diverse cancer cells in a tumor are treated as though they have unlimited proliferative potential and can acquire the ability to metastasize. For many years, however, it has been recognized that small numbers of disseminated cancer cells can be detected at sites distant from primary tumors in patients that never manifest metastatic disease. One possibility is that most cancer cells lack the ability to form a new tumor such, that only the dissemination of rare cancer stem cells can lead to metastatic disease. Hence, the goal of therapy must be to identify and kill this cancer stem cell population.
Existing therapies have been developed largely against the bulk population of tumor cells, because the therapies are identified by their ability to shrink the tumor mass. However, because most cells within a cancer have limited proliferative potential, an ability to shrink a tumor mainly reflects an ability to kill these cells. Therapies that are more specifically directed against cancer stem cells may result in more durable responses and cures of metastatic tumors.
The existence of cancer stem cells (CSCs) have been demonstrated in a variety of human cancers. These CSCs possess the capacity for self-renewal, differentiation and display resistance to chemotherapeutic agents and radiation, which may be the cause of tumor relapse years after the clinical remission (2, 3).
Recent evidence indicates that cells within a tumor are heterogeneous and represent different stages of development (Clarke et al. 2006. Cancer Res. 66:9339-9344). In certain types of cancer, a population of cells has been identified that are termed cancer stem cells, where a cancer stem cell is defined as a cell that has the capacity to self-renew and to cause the heterogeneous lineages of cancer cells that comprise a tumor. Experimentally, such cells are ones that have the ability to generate a continuously growing tumor (Clarke et al. 2006. Cancer Res. 66:9339-9344). Cancer stem cells can arise from normal stem cells but also from cells that acquire the capacity to self-renew potentially due to a series of mutagenic events within the cell. There is considerable interest in the role of cancer stem cells in certain types of cancer. Cancer types that have been associated with the presence of cancer stem cells include breast cancer (Al-Hajj et al. 2003. PNAS 100:3983-3988), pancreatic cancer (Hermann et al. 2007. Cell Stem Cell 1:313-323), brain cancer (Singh et al. 2004. Nature 432:396-401), and testicular cancer (Houldsworth et al. 2006. J. Clin. Oncol. 24:5512-5518; Clark A. T. 2007. Stem Cell Rev. 3:49-59.
To design therapy against cancer, it is desirable to seek molecular targets of cancer or cancer stem cells that are absent from normal cells. Although many chemotherapeutic drugs and targeted therapeutic agents have improved the cure rate of cancer, recurrent disease remains to be a challenge due to the development of drug resistance. Thus, it will be important to identify biosignature for early detection, prognostication and prediction of therapeutic responses for cancer. So far, there is no reliable and validated biomarkers specific for early detection of cancer.
It is highly desirable to be able to identify these cancer stem cells using specific markers, and then use these markers to develop cancer stem cell specific therapeutics. The present invention addresses this issue.